Intra-aortic balloon pumping has been completed in small animals weighing from 3.1 kilograms to 4.8 kilograms. The preliminary data indicate that relative sized balloons (0,75 ml to 2.0 ml) can be employed successfully as a means of circulatory assist. There has been no difference between the performance of tapered and non-tapered balloons. Testing with the Utah Heart Driver, the modified AVCO 10 intra-aortic balloon pump, and the Datascope Model 80 and 82 intra-aortic balloon pump indicate greater flexibility and superior performance with the Datascope machine for possible pediatric use. Modes of introduction of the intra-aortic balloon pump will be tested further in small animals and will be completed during the coming year. The goal is to apply this mode of circulatory assist in a clinical setting in the next six to eight months.